Ion exchange assembly for an electrochemical cell

ABSTRACT

This invention provides a solid polymer ion exchange membrane/electrode assembly, or an electrode/solid polymer ion exchange membrane/electrode assembly, for an electrochemical cell, which consists of planar layers of materials intimately joined together to form a unitary structure. The layers are joined together by solid polymer ion exchange resin present in at least one of each pair of adjacent layers. The unitary assembly can be used in an electrochemical cell such as a battery, electrolytic reactor, or fuel cell.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application under 37 C.F.R.j 1.53(b) ofapplication Ser. No. 09/405,134, filed Sep. 24, 1999 now abandoned,which is a continuation of Ser. No. 09/200,155 filed Nov. 25, 1998 nowabandoned, which is a continuation of Ser. No. 09/016,204, filed Jan.30, 1998 now abandoned, which is a continuation of Ser. No. 08/568,100,filed on Dec. 6, 1995, now U.S. Pat. No. 6,054,230.

FIELD OF THE INVENTION

This invention relates to ion exchange membranes and electrodes for usein electrochemical devices such as batteries, fuel cells, andelectrolytic reactors. More particularly, the invention relates toelectrodes and solid polymer ion exchange membranes combined to form aunitary assembly.

BACKGROUND OF THE INVENTION

Electrochemical cells which employ ion exchange membranes formed ofsolid polymer ion exchange resins and electrodes in whichcatalytically-active electrically-conductive materials are included arewell known in the art. Such cells can be used for the generation ofelectricity, for example, in fuel cells and batteries; or inelectrolytic reactors, for example, for electrolysis of water, chemicalsynthesis, and many other uses.

Such cells are manufactured by various techniques which provide astructure of a solid polymer electrolyte (SPE) membrane, or protonexchange membrane, for ion exchange, sandwiched between electrodes forcurrent transfer and, in the case of gaseous fuel cells, gas diffusion.Solid polymer ion exchange membranes useful in such devices can beselected from commercially available membranes, for example,perfluorinated membranes sold under the tradenames Nafion® (DuPont Co.)and Flemion® (Asahi Glass Co.); or formed as films cast from solutionscontaining commercially available ion exchange resins. The electrodesare often formed of electrically-conductive particulate materials (whichmay include catalyst materials) held together by a polymeric binder.Polytetrafluoroethylene (PTFE) resin, due to its chemical inertness andhigh temperature resistance, is often used as the polymeric binder. ThePTFE resin is usually combined with the particulate electrode materialsand molded or processed into sheet form by PTFE paste-forming processesknown in the art. The cells may also include porous current collectionor distribution layers, for example, platinum wire mesh or woven carboncloth, in contact with the electrode surfaces facing away from the ionexchange membrane. Important considerations in such layered structuresinclude uniformity in the thickness and distribution of functionalmaterials forming, or within, the layers; and quality and durability ofcontact between the layers. It is also desired that the layers be asthin as possible to increase the energy efficiency and current densityof the cells.

To improve the energy efficiency of the electrochemical devices,electrode structures have been modified to increase the number ofreaction sites. In addition, to increase the rate of ion movement, solidpolymer ion exchange resins have been included within the electrodestructures. To allow the ions produced to move rapidly toward thecounter electrode, it is necessary to improve the contact between thesolid polymer ion exchange resin inside the electrode and the ionexchange membrane, and to lower the membrane resistance of the ionexchange membrane itself.

Conventionally, the solid polymer membranes are joined to the electrodesby hot pressing or simply held together in the cell by mechanical forcesapplied to them. It is difficult, however, to produce a cell with thinmembranes by either method. When hot pressing is used the membranematerial is softened and weakened by the heat and, if too thin, willrupture and create a gas leakage path or cause a short circuit betweenthe electrodes. Such problems are exacerbated if the electrode surfaceshave poor smoothness. When only mechanical force is used, a much greaterforce is required to ensure uniform contact and to obtain a low contactresistance between the membrane and electrodes, and the same problemsare encountered with thin ion exchange membranes. A further disadvantageis that pressure applied to force the electrodes and membrane together,whether with or without heat, can cause compaction of the electrodes andthus reduce the gas permeability of the electrode.

Means used to address these problems include applying and drying asolution containing a solid polymer ion exchange resin to an electrodesurface, and then joining the coated electrode to an ion exchangemembrane by hot pressing. Another method described is to apply asolution containing a solid polymer ion exchange resin, or a solvent forthe resin, to an electrode surface and then, with solvent still presenton the surface, join the coated electrode to an ion exchange membrane,after which the solvent is removed. In another method a solutioncontaining a solid polymer ion exchange resin is applied to a surface oftwo electrodes and, while still wet, the coated surfaces are broughttogether, after which the solvent is removed and an ion exchangemembrane formed between the electrodes. These methods, too, sufferdrawbacks in that it is very difficult to control the penetration of theapplied solutions into the electrodes and excessive amounts of thesolutions must be applied. This often results in impaired gasdiffusitivity in the electrodes and also makes it difficult to obtain athin ion exchange membrane with a uniform thickness.

Other methods to produce ion exchange membrane/electrode structures inwhich electrodes are formed on a current collector and subsequentlyjoined to an ion exchange membrane; or in which electrodes are formeddirectly on an ion exchange membrane and subsequently joined to a gasdiffusion material or current collector, are also known in the art. Mostsuch methods are variants or combinations of the methods describedabove, except that different substrates are used, and have drawbackssimilar to those described.

U.S. Pat. No. 5,234,777 (to Wilson) is for a membrane catalyst layerstructure for a fuel cell which incorporates a thin catalyst layerbetween a solid polymer electrolyte and a porous electrode backing.Wilson discloses a catalyst film formed from an ink preparationconsisting of a mixture of carbon particle-supported platinum catalyst,a solubilized ion exchange resin, and thickening agents. The electrodeink can be applied to a release surface, oven-dried to form a thinlayer, and, after sufficient layers have been added to form the film,removed and hot pressed to an ion exchange membrane. An alternativemethod is also disclosed in which a different form of the ion exchangeresin is solubilized in the ink mixture, the electrode ink is applied tothe surface of an ion exchange membrane, heated and dried to form alayer, and, after sufficient layers have been added to form the film,treat the assembly to convert the ion exchange resin to its use form.

SUMMARY OF THE INVENTION

This invention provides an electrode/solid polymer ion exchange membraneassembly for an electrochemical cell comprising planar layers ofmaterials intimately joined together to form a unitary structure. Thelayers are intimately joined together by a bond, formed across the layerinterface, by solid polymer ion exchange resin present in at least oneof each pair of adjacent layers. Each embodiment of the invention hasone or more layers supported by at least one preformed support matrixformed of porous polytetrafluoroethylene. The preformed support matrixof polytetrafluoroethylene contains the electrode-forming or ionexchange membrane-forming materials of the layer, and provides strength,reinforcement, and handleability, while substantially preventingmigration of the materials into adjacent layers.

A planar article or form, as used herein, is an article or form made soas to have length and width dimensions, or radial dimensions, muchgreater than the thickness dimension. Examples of such articles includea polymeric film or membrane, a sheet of paper, a textile fabric, aribbon, or a disc, and the like. It is apparent that, once formed, sucharticles can be used as an essentially flat article, or wound, folded,or twisted into more complex configurations.

By porous as used herein is meant a structure of interconnected pores orvoids such that continuous passages and pathways throughout a materialare provided.

One embodiment of the invention is a unitary assembly which comprises aplanar composite solid polymer ion exchange membrane comprising at leastone preformed membrane-support of porous polytetrafluoroethylene whichcontains, and is made nonporous by, solid polymer ion exchange resin;and a planar electrode in intimate contact with and bonded to a planarsurface of the solid polymer ion exchange membrane by the solid polymerion exchange resin.

Another embodiment of the invention is a unitary assembly whichcomprises a planar composite solid polymer ion exchange membranecomprising at least one preformed membrane-support of porouspolytetrafluoroethylene which contains, and is made nonporous by, solidpolymer ion exchange resin; and two planar electrodes, each electrode inintimate contact with and bonded to a planar surface of the solidpolymer ion exchange membrane by the solid polymer ion exchange resin.

Yet another embodiment of the invention is a unitary assembly whichcomprises a planar composite solid polymer ion exchange membranecomprising at least one preformed membrane-support of porouspolytetrafluoroethylene which contains, and is made nonporous by, solidpolymer ion exchange resin; and a planar electrode comprising apreformed electrode-support of porous polytetrafluoroethylene containingboth a solid polymer ion exchange resin and a catalyst material, andwhich is in intimate contact with and bonded to a planar surface of thesolid polymer ion exchange membrane by solid polymer ion exchange resin.

A further embodiment of the invention is a unitary assembly whichcomprises a planar composite solid polymer ion exchange membranecomprising at least one preformed membrane-support of porouspolytetrafluoroethylene which contains, and is made nonporous by, solidpolymer ion exchange resin; and two planar electrodes each of whichcomprises a preformed electrode-support of porouspolytetrafluoroethylene containing both a solid polymer ion exchangeresin and a catalyst material and which are in intimate contact with andbonded to a planar surface of the solid polymer ion exchange membrane bysolid polymer ion exchange resin.

Other embodiments of the invention are unitary assemblies which comprisea planar solid polymer ion exchange membrane and one or two planarelectrodes; each electrode comprising a preformed electrode-support ofporous polytetrafluoroethylene containing both a solid polymer ionexchange resin and a catalyst material, and each electrode in intimatecontact with and bonded to a planar surface of the solid polymer ionexchange membrane by solid polymer ion exchange resin.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a unitary solid polymer ion exchangemembrane/electrode assembly. The solid polymer ion exchange membranecomprises at least one preformed membrane-support of porous expandedpolytetrafluoroethylene which is filled and made nonporous with solidpolymer ion exchange resin. The porous PTFE membrane-support istypically filled by impregnation of a liquid composition containing theion exchange resin into the membrane-support. The electrode compriseselectrode constituents, which include both a solid polymer ion exchangeresin and a catalyst material, and a support matrix ofpolytetrafluoroethylene. The electrode is intimately joined and adheredto the solid polymer ion exchange membrane by a bond formed between theion exchange resin present in both layers.

As noted above, in the unitary solid polymer ion exchangemembrane/electrode assembly of the invention, the composite ion exchangemembrane comprises at least one membrane-support consisting of apreformed porous film of polytetrafluoroethylene. Electrodes of theassembly preferably also comprise an electrode-support consisting of apreformed porous film of polytetrafluoroethylene. However, otherelectrode structures, also inclusive of a polytetrafluoroethylenesupport matrix, preferably an expanded polytetrafluoroethylene supportmatrix, can be obtained by mixing together electrically-conductiveparticulate materials and PTFE resin particles and co-forming themixture to produce an electrode structure suitable for use in theassembly. Such methods are particularly useful for forming gas diffusionregions or gas diffusion layers of the electrodes. A process forco-forming particulate materials and polytetrafluoroethylene resin toproduce a particle-filled expanded polytetrafluoroethylene film isdisclosed in U.S. Pat. No. 4,985,296 (to Mortimer).

Porous polytetrafluoroethylene film suitable for use as themembrane-support or electrode-support can be made by processes known inthe art, for example, by stretching or drawing processes, by papermakingprocesses, by processes in which filler materials are incorporated withthe PTFE resin which are subsequently removed to leave a porousstructure, or by powder sintering processes. Preferably the porouspolytetrafluoroethylene film is porous expanded polytetrafluoroethylenefilm having a structure of interconnected nodes and fibrils, asdescribed in U.S. Pat. Nos. 3,953,566 and 4,187,390 (to Gore), andEuropean Patent Application No. 0 661 336 (to Morishita, et al.), whichdescribe the preferred material and processes for making them.

For use as a membrane-support, the porous polytetrafluoroethylene filmshould have a thickness in the range 1 to 100 micrometers, preferably inthe range 2 to 30 micrometers; a pore volume in the range 60 to 98percent, preferably in the range 80 to 95 percent; and a nominal poresize in the range 0.05 to 5 micrometers, preferably in the range 0.2 to2 micrometers. A membrane which is too thin tends to have flaws whichcause short circuits or gas leaks across the membrane. If the pore sizeis too small, impregnation of the solid polymer ion exchange resin intothe membrane-support is made difficult; and if the pore size is toolarge the membrane-support loses its ability to retain and preventmigration of the liquid composition containing the solid polymer ionexchange resin. Too low a pore volume increases the membrane resistancedue to an insufficiency of ion exchange resin; and an excessively highpore volume results in a membrane-support film too weak for use.

For use as an electrode-support, the porous polytetrafluoroethylene filmshould have a thickness in the range 3 to 200 micrometers, preferably inthe range 5 to 20 micrometers; a pore volume in the range 60 to 95percent, preferably in the range 85 to 95 percent; and a maximum poresize defined by an isopropanol bubble point (IBP) in the range 0.05 to0.5 kg/cm², preferably in the range 0.05 to 0.3 kg/cm². (A descriptionof the Bubble Point Test is provided hereinbelow). The optimumelectrode-support film thickness varies according to the amount ofcatalyst material needed for an application, but a thickness below about3 micrometers makes it difficult to obtain an adequate number and properthree-dimensional distribution of catalyst sites. A thickness greaterthan about 200 micrometers impedes gas diffusion and ion-conduction, andthe electrode cannot function properly. A pore volume less than about 60percent increases the amount of PTFE relative to the amount of catalystmaterial and does not permit low resistance values to be obtained. Amaximum pore size indicated by an IBP greater than 0.5 kg/cm²corresponds to a porous PTFE structure having pore sizes too small toeasily introduce catalyst materials, e.g., Platinum-supporting carbonparticles, into the structure. It is also preferred that the structureof the porous expanded polytetrafluoroethylene electrode-support, i.e.,a structure of nodes interconnected by fibrils, be one in which the sizeof the nodes is as small as possible in order to increase the usablepore volume and facilitate introduction of the catalyst particles andsolid polymer ion exchange resin into the structure.

It is through the use of such independently made preformed expandedpolytetrafluoroethylene support membranes that the performance of theion exchange membrane/electrode assembly can be optimized. Membraneproperties such as strength, pore volume, pore size, and thickness canbe tailored according to the needs of the layer of the compositeassembly in which they will be used to obtain desired cell properties,such as catalyst amounts, gas diffusivity, electronic and ionicconduction. Furthermore, use of the membrane-support andelectrode-support films provides much greater uniformity andreproducibility, permits a variety of manufacturing methods to be used,and thus, greater flexibility in choice of manufacturing methods, andfacilitates manufacture of the assembly. Additionally, their presenceduring manufacture of the assembly serves to reduce problems associatedwith migration of solvents or other materials into adjacent layers suchas are encountered in the manufacture of conventional electrochemicalcell structures.

No particular limitations are placed on the ion exchange or electrolyteresins so long as they are amenable to impregnation into and retentionby the membrane-support and electrode-support films. Hydrocarbon-basedor fluorine-based ion exchange resins can be used as desired.Preferably, solid polymer ion exchange resins are used, in particular,solid polymer ion exchange resins which can be dissolved, or at leastpartially dissolved, in suitable solvents to form liquid compositionsthat can be impregnated into the support films. Most preferable areperfluorocarbon-based ion exchange resins, especially perfluorosulfonicacid resins, for example, such as are sold under the trademarks Nafion®(DuPont Co.) or Flemion® (Asahi Glass Co.). Suitable solvents for theseion exchange resins are well known in the art, and include variousalcohols and other organic solvents, water, or mixtures of these withwater. To aid in impregnation of a porous expandedpolytetrafluoroethylene membrane-support film, depending on themolecular weight of the solid polymer resin or on the type of solventused, the solvent concentration in the liquid composition can be varied,a surface treatment given to the polytetrafluoroethylene, or asurfactant used. Surfactants may also be included in the liquids tofacilitate mixing and dispersion of the materials forming the liquidcompositions.

As with the ion exchange resins, no particular limitations are placed onthe catalyst materials so long as they are amenable to impregnation intoand retention by the membrane-support and electrode-support films. Anyparticulate material, or powder, acting as a catalyst can be used, andwill be selected according to the application intended. Examplesinclude, but are not limited to, lead dioxide for ozone generatingelectrodes, platinum or platinum alloys for hydrolytic electrodes,platinum or platinum alloys supported on carbon black, and the like.

The catalyst materials and a solid polymer ion exchange resin arecombined in a liquid mixture for impregnation into an electrode-supportfilm, or for surface coating or impregnation into an electrodestructure. This can be done, for example, by dispersing the catalystpowder in solvents such as those described above, and then adding ionexchange resin, or a liquid composition containing ion exchange resin,to form the liquid mixture. If desired, it is also possible to include afluoropolymer, such as PTFE, tetrafluoroethylene/(perfluoroalkyl) vinylether copolymer (PFA), or tetrafluoroethylene/hexafluoropropylenecopolymer (FEP), in such liquid mixtures to enhance water repellency inthe electrode structure. It is also possible to include a pore-formingagent, such as ammonium bicarbonate, sodium chloride, or calciumcarbonate, which is removed after formation of the membrane, forexample, by heating or leaching, to create voids to improve gasdiffusivity.

Catalyst materials can also be introduced into an electrode structure asa catalyst precursor. In such a case the liquid mixture to beimpregnated into an electrode-support film is a mixture obtained bycombining a liquid dispersion of noncatalytic electrically-conductiveparticles and a liquid composition containing the catalyst precursor anda solid polymer ion exchange resin. That is, it may be a liquid mixtureof noncatalytic electrically-conductive particles, a solid polymer ionexchange resin, and a solid polymer ion exchange resin which has acatalyst metal precursor bonded to its exchange groups. For example,carbon black is used as the electrically-conductive particles; thecarbon black is dispersed in a liquid composition containing solidpolymer ion exchange resin to allow the resin to adsorb onto the carbonblack. Catalyst metal anions, such as in a platinum-ammine complexsolution, are then added to bring about ion exchange, after which moresolid polymer ion exchange resin is added. The ingredients can be mixedsimultaneously or added sequentially. When such a mixture is used, thecatalyst precursor must be converted to a catalyst by some type ofreducing treatment after the solid polymer ion exchangemembrane/electrode has been formed. Such reducing treatments includeheating and hydrogen reduction, chemical reduction using sodiumborohydride, and other reducing treatments known in the art. A highlyactive catalyst can be obtained with the use of such methods.

In the preparation of a unitary ion exchange membrane/electrode assemblyof the present invention, the pores of the porous expandedpolytetrafluoroethylene membrane-support film are impregnated with asolid polymer ion exchange resin to obtain a composite membrane that isthin yet has high strength. Impregnation can be accomplished usingequipment and methods known in the art, and no particular restrictionsare imposed. For example, the porous expanded polytetrafluoroethylenemembrane-support film can be dipped or immersed in a liquid compositioncontaining the resin; or the liquid composition may be applied to thesurface by brushing or spraying, by screen printing, by roll coating,and the like, after which the solvent is removed. Such methods may berepeated a number of times until the pores are essentially completelyfilled with the solid polymer ion exchange resin and a nonporouscomposite film is produced. The solvent can be removed by any convenientmethod such as air drying, heating in an oven or over heated rolls, andthe like. If heating is used, temperatures which can lead todecomposition of the ion exchange resin should be avoided. Due to thestrength and handleability of the porous expandedpolytetrafluoroethylene film, and its ability to retain the liquidcomposition containing the ion exchange resin in its porous structure,the composite solid polymer ion exchange resin-filled membrane-supportfilm can be formed separately, and subsequently intimately joined to anelectrode structure; or it can also be formed in place on the surface ofan electrode or other substrate, for example, by first superposing theporous expanded polytetrafluoroethylene membrane-support film on anelectrode structure and then impregnating the membrane-support film,which simultaneously joins it to the electrode.

A preferred structure for the unitary solid polymer ion exchangemembrane/electrode assembly of the invention comprises an electrodestructure having a preformed electrode-support also consisting of apreformed porous film of polytetrafluoroethylene. The porous expandedpolytetrafluoroethylene electrode-support films are impregnated with theliquid mixtures containing catalyst materials and solid polymer ionexchange resin by the same means described above. As with the ionexchange membrane-support described above, the electrode-support filmcan be impregnated separately; or while on a substrate providing arelease surface, or on the surface of a substrate to which it issimultaneously intimately joined, such as, for example, the surface of acollector, a gas diffusion material, an ion exchange membrane, orpreferably, a composite solid polymer ion exchange resin-filledmembrane-support film.

In the course of impregnation and desolvation/solidification of theliquid mixtures, the solid polymer ion exchange resin causes thecatalyst particles to adhere to each other and serves as a binder infixing the catalyst particles in the internal structure of the expandedpolytetrafluoroethylene matrix. The solid polymer ion exchange resin inthe electrode structure thus formed also serves as a binder tointimately join the structure to a composite solid polymer ion exchangeresin-filled film described above, and further serves to form routesthrough which the ions produced over the catalyst particles rapidlymigrate to the solid polymer ion exchange membrane. The mechanicalstrength of the electrode structure is derived from the expandedpolytetrafluoroethylene electrode-support matrix, therefore, only enoughsolid polymer ion exchange resin to accomplish the above purposes needbe used, and results in a structure with good gas diffusibility. Anexcess quantity of solid polymer ion exchange resin is not needed forthe development of strength properties in thepolytetrafluoroethylene-supported structure and, because an excessivequantity of resin may reduce gas permeability as well as complicatemanufacturing, is undesirable. Furthermore, when solvent is removed fromthe impregnated liquid mixture, the solid components aggregate, with anattendant decrease in volume, however, the aggregating force isaccommodated and distributed by the expanded polytetrafluoroethylenematrix so that minute cracks and gaps are formed in the contractingsolids, further enhancing gas diffusivity.

As noted earlier, use of preformed porous expandedpolytetrafluoroethylene films makes possible a variety of manufacturingmethods of the unitary solid polymer ion exchange membrane/electrodeassembly of the invention. When either a supported composite ionexchange structure or composite electrode structure is made separately,it can be joined to the other by conventional methods such as byapplication of heat and pressure. However, a preferred method of joiningthe structures is by solvent-aided adhesive methods in a manner suchthat the influence of the solvent, or ion exchange resin and solventused as an adhesive, is limited to the region near the junction of thelayers to be joined. The retention characteristics of the expandedpolytetrafluoroethylene support matrix substantially prevents migrationof solid polymer ion exchange resin, even when softened by a solvent,from the supported composite ion exchange structure, and likewise,substantially prevents migration of electrode materials from a supportedcomposite electrode structure. The solvent-aided joining of the layerscan be effected, as indicated above, when one of the supported compositestructures is impregnated and formed on the surface of anotherpreviously made structure. In the case where both structures to bejoined have been separately made and the solvent completely removed, alight topical application of solvent, or of a liquid composition of asolvent containing solid polymer ion exchange resin, can be made to thesurface to be joined of a supported composite structure, and the layersjoined. An advantage of such solvent-aided adhesion is that thestructure can be joined with minimal compressive force applied to themand are not deformed in the joining step. In instances where a compositestructures is joined to another as it is impregnated, virtually nocompressive force is applied. When a separately made and dried supportedcomposite structure is joined to another, only a light compressive forceapplied in a manner to prevent formation and entrapment of air bubblesbetween the layers is needed.

It is apparent from the foregoing that the unitary assembly of theinvention can be formed by a variety of methods and having a number ofstructures. For example, a unitary assembly having a composite solidpolymer ion exchange resin-filled membrane-support film bonded to eachside of a previously made ion exchange membrane; a unitary assemblyhaving a composite solid polymer ion exchange resin-filledmembrane-support film, on one or both sides of which is bonded anelectrode structure; and, preferably, a unitary assembly having acomposite solid polymer ion exchange resin-filled membrane-support filmon one or both sides of which is bonded an electrode structure having acomposite solid polymer ion exchange resin-filled electrode-supportfilm.

TEST METHODS

Bubble Point Test

The Bubble Point was measured according to the procedures of ASTMF316-86. Isopropyl Alcohol was used as the wetting fluid to fill thepores of the test specimen.

The Bubble Point is the pressure of air required to displace theisopropyl alcohol from the largest pores of the test specimen and createthe first continuous stream of bubbles detectable by their rise througha layer of Isopropyl Alcohol covering the porous media. This measurementprovides an estimation of maximum pore size.

EXAMPLE 1

An electrode consisting of graphite particles (95 wt. %) and PTFE resinparticles (5 wt. %) was prepared by conventional paste-forming methodsin which the particulate materials were mixed together, lubricated,ram-extruded to form a tape, and calendered to form an electrode sheetfor a lithium ion cell.

The surface of the electrode sheet was lightly coated with a solution ofan alkylene oxide polymer oligomer containing 0.15 mol of lithiumperchlorate per ether linkage of the oligomer and 1 wt. % (based on thetotal weight of oligomer and lithium perchlorate) of benzyl dimethylketone, an ultraviolet (UV) radiation activated crosslinking agent.

A porous expanded polytetrafluoroethylene film about 3 micrometers thick(Gore-Tex® expanded PTFE film, manufactured by Japan Gore-Tex, Inc.),and having a nominal pore size of about 1 micrometer and pore volume of93 percent was fixed to the surface of the coated electrode sheet. Theporous PTFE film was coated and impregnated with the same solutionapplied to the electrode sheet so as to essentially completely fill thepores of the PTFE film and contact the solution coated on the electrodesheet, after which the composite article was subjected to UV radiationto effect crosslinking, and a unitary solid polymer ion exchangemembrane/electrode assembly of the invention was produced.

EXAMPLE 2

A gas diffusion electrode for a fuel cell was prepared as follows:

An aqueous dispersion of carbon black particles (“Denka Black”, suppliedby Denka Co.) and PTFE resin particles having a solids concentration 65wt. % carbon black and 35 wt. % PTFE was prepared. The PTFE wascoagulated, and the coagulum of mixed carbon black and PTFE dried.Naphtha was added and mixed into the dried coagulum as a lubricant. Thelubricated coagulum was ram-extruded to form a tape 2.5 mm thick. Theextruded tape was calendered and the thickness reduced to 250micrometers. The calendered tape was uniaxially stretched (in thelongitudinal direction) at a temperature of about 250° C. to 5 times itsoriginal length, and then again calendered to reduce its thickness by afactor of 5. The porous electrically-conductive gas permeable electrodesheet thus produced was about 50 micrometers thick, had a nominal poresize of about 1 micrometer, and a pore volume of about 78%.

A collector sheet consisting of 130 micrometer thick carbon paper,supplied from Toray Co., was impregnated with an aqueous dispersion ofPTFE. The PTFE-treated collector sheet and gas diffusion electrode sheetwere laminated together by application of heat (120° C.) and pressure(20 kg/cm²), after which the laminated assembly was heat treated at 360°C. for 10 minutes.

A liquid mixture containing catalyst material and solid polymer ionexchange resin was prepared. The catalyst material was platinum-coated(25 wt. %) carbon black (tradename—Vulcan® XC72), and the solid polymerion exchange resin was Nafion® perfluorosulfonic acid resin(manufactured by DuPont Co.). A dispersion of 5 grams of Pt-coatedcarbon black in 40 grams of 2-methyl, 1-propyl alcohol was prepared. Aliquid composition of isopropyl alcohol containing 9 wt. % Nafion®perfluorosulfonic acid resin was added to the dispersion to provide aliquid mixture having a relative concentration of 30 wt. %perfluorosulfonic acid resin and 70 wt. % Pt-coated carbon. The liquidmixture was applied by brush to the surface of the gas diffusionelectrode sheet, thereby forming a solid polymer ion exchangeresin/catalyst containing region, and the solvent removed, thuscompleting the electrode structure.

A porous expanded polytetrafluoroethylene film was fixed on the solidpolymer ion exchange resin/catalyst coated surface of the electrode. ThePTFE film was 20 micrometers thick, had a nominal pore size of 0.2micrometer, and a pore volume of 89%. The porous PTFE film was coatedwith a liquid composition of isopropyl alcohol containing 5 wt. %Nafion® perfluorosulfonic acid resin (manufactured by DuPont Co.), andair dried. The coating and air drying steps were repeated 5 times untilthe pores of the PTFE film were essentially completely filled, thelayers joined by the solid polymer ion exchange resin present in eachlayer, the composite membrane-support film became semitransparent, andthe surface of the film coated with the solid polymer ion exchangeresin. The composite assembly was heat treated at 130° C. for 24 hours,and a unitary solid polymer ion exchange resin/electrode assembly wasobtained.

A second unitary solid polymer ion exchange resin/electrode assembly wasobtained exactly as described above. The membrane-supported ion exchangeresin surface of one of the assemblies was coated with a liquidcomposition of isopropyl alcohol containing 2 wt. % Nafion®perfluorosulfonic acid resin, placed on the membrane-supported ionexchange resin surface of the second assembly and lightly pressed toremove entrapped air, after which the solvent was removed by air dryingand the joined assemblies heat treated at 130° C. for 24 hours.

The unitary assemblies thus joined formed a larger unitary embodiment ofthe invention to which further components were joined. The completeassembly described above was mounted and operated as a gaseous fuel cellHumidified hydrogen was fed on one side of the mounted assembly, andoxygen was fed on the other side at an operating temperature of 80° C.The cell developed a voltage of 0.78 volts at a current level of 1A/cm².

Comparative Example 1

An electrochemical cell assembly was prepared as described in Example 2,except that no membrane-support films were used and the liquidcomposition of isopropyl alcohol containing Nafion® perfluorosulfonicacid resin was applied directly to the gas permeable electrode sheet.Numerous cracks formed, and partial separation from the substrateoccurred.

The electrochemical cell assembly was tested in a fuel cell as describedin Example 2, and developed a voltage of 0.67 volts at a current levelof 1 A/cm².

EXAMPLE 3

A composite membrane-supported solid polymer ion exchange resin-filledfilm was prepared separately. The PTFE film was 15 micrometers thick,had a nominal pore size of 0.2 micrometer, and a pore volume of 89%.

The porous expanded polytetrafluoroethylene membrane-support film wasmounted in an open frame which gripped the film at the edges to restrainIt from shrinking during impregnation of a liquid composition ofisopropyl alcohol containing 5 wt. % solid polymer ion exchange resin(Nafion® perfluorosulfonic acid resin). The restrained film was coatedwith the liquid composition which was absorbed by the porousmembrane-support film, and air dried. The coating and drying steps wererepeated three times, the pores of the PTFE film were essentiallycompletely filled, and a semitransparent film was produced

A small amount of the same liquid composition was then applied by brushto the surface of the Nafion® resin-filled membrane-support film thusproduced and the wetted surface placed on the solid polymer ion exchangeresin/catalyst coated surface of an electrode structure, made asdescribed in Example 2, and lightly pressed to remove entrapped air andintimately join the film to the electrode structure. The solvent wasremoved by air drying, thus completing a unitary assembly of theinvention.

EXAMPLE 4

A composite membrane-supported solid polymer ion exchange resin-filledfilm was prepared separately, as described in Example 3.

A small amount of solvent, isopropyl alcohol, was applied by brush tothe solid polymer ion exchange resin/catalyst coated surface of anelectrode structure (made as described in Example 2). the Nafion®resin-filled membrane-support film was immediately placed on the wettedsurface of the electrode structure and lightly pressed to removeentrapped air and intimately join the film to the electrode structure.The solvent was removed by air drying, thus completing a unitaryassembly of the invention.

EXAMPLE 5

A composite membrane-supported solid polymer ion exchange resin-filledfilm was prepared as described in Example 3, except that the porousexpanded polytetrafluoroethylene membrane-support film was 30micrometers thick.

A small amount of a liquid composition of isopropyl alcohol containing 5wt. % solid polymer ion exchange resin (Nafion® perfluorosulfonic acidresin)solvent, isopropyl alcohol, was applied by brush to both surfacesof the resin-filled film and the film was sandwiched between the solidpolymer ion exchange resin/catalyst coated surfaces of two electrodestructures (made as described in Example 2). The assembly was lightlypressed together to remove entrapped air and intimately join the film tothe electrode structures. The solvent was removed by air drying, thuscompleting a unitary assembly of the invention.

EXAMPLE 6

A porous expanded polytetrafluoroethylene film was fixed on the solidpolymer ion exchange resin/catalyst coated surface of an electrodestructure (made as described in Example 2). The PTFE film was 40micrometers thick, had a nominal pore size of 0.7 micrometer, and a porevolume of 92%. The porous PTFE film was coated with a liquid compositionof isopropyl alcohol containing 5 wt. % Nafion® perfluorosulfonic acidresin (manufactured by DuPont Co.), and air dried. The coating and airdrying steps were repeated 3 times until the pores of the PTFE film weresubstantially filled. A fourth coating step was conducted, the solidpolymer ion exchange resin/catalyst coated surface of a second electrodestructure was lightly pressed against the wetted surface and joined bythe solid polymer ion exchange resin present in each layer. The solventwas removed by air drying, thus completing a unitary assembly of theinvention.

EXAMPLE 7

A liquid dispersion of fine lead dioxide particles (0.2 micrometerparticle size) in isopropyl alcohol was prepared. To the dispersion wasadded a liquid composition of isopropyl alcohol containing 5 wt. %Nafion® perfluorosulfonic acid resin (manufactured by DuPont Co.) andthoroughly mixed to form a first liquid mixture, having a relativeconcentration of 15 wt. % ion exchange resin and 85 wt. % lead dioxide.

A porous expanded polytetrafluoroethylene electrode-support film(thickness—12 micrometers; pore volume—93%; IBP-0.08 kg/cm²) wassuperposed on a polypropylene release sheet and passed through the nipof a roll coater. The first liquid mixture described above wasroll-coated on the surface of the electrode-support film and forced intothe pores of the electrode-support film, after which the solvent wasremoved by air drying, the impregnated electrode-support film removedfrom the release sheet, and a first electrode thus completed.

A dispersion of carbon black/platinum (20 wt. %) particles (from NEChemcat Co.) in isopropyl alcohol was prepared. To the dispersion wasadded a liquid composition of isopropyl alcohol containing 5 wt. %Nafion® perfluorosulfonic acid resin (manufactured by DuPont Co.) andthoroughly mixed, with the aid of ultrasonic agitation, to form a secondliquid mixture, having a relative concentration of 40 wt. % ion exchangeresin and 60 wt. % carbon black supported platinum.

A second porous expanded polytetrafluoroethylene electrode-support film,identical to the first electrode-support, was fixed to the surface of200 micrometers thick carbon paper (manufactured by Toray Co.). Thesecond liquid mixture was coated on the surface of the electrode-supportfilm and impregnated into the pores of the film, after which the solventwas removed by air drying, and a second electrode thus completed.

A solid polymer ion exchange membrane (Nafion® 117 perfluorosulfonicacid membrane, manufactured by DuPont. Co.) was sandwiched between thefirst electrode structure and the second electrode structure, andlaminated by application of heat (140° C.) and pressure (25 kg/cm²) toform a unitary assembly of the invention.

A platinum-plated titanium mesh was applied to the surface of the firstelectrode as a collector, and the unitary assembly and collectorsandwiched between ribbed, platinum-plated, stainless steel plates toform an electrochemical cell. Purified water was fed to the ribbedportions, and the cell was operated as an ozone generator by waterelectrolysis using a solid polymer electrolyte.

EXAMPLE 8

A dispersion of 5 grams of carbon black/platinum (25 wt. %) particles(from NE Chemcat Co.) in 40 grams of 2-methyl, 1-propyl alcohol wasprepared. To the dispersion was added a liquid composition of isopropylalcohol containing 9 wt. % Nafion® perfluorosulfonic acid resin(manufactured by DuPont Co.) and thoroughly mixed, with the aid ofultrasonic agitation, to form a liquid mixture, having a relativeconcentration of 25 wt. % ion exchange resin and 75 wt. % carbon blacksupported platinum.

A collector sheet consisting of 230 micrometer thick carbon paper,manufactured by Toray Co., was impregnated with an aqueous dispersion ofPTFE to develop water repellency, and then heat treated at 360° C. for10 minutes. A porous expanded polytetrafluoroethylene electrode-supportfilm (thickness—16 micrometers; pore volume—94%; IBP-0.12 kg/cm²) wasfixed to the surface of the carbon paper. The liquid mixture was appliedby brush to impregnate the pores of the electrode-support film, afterwhich the solvent was removed by air drying. The composite structure washeat treated at 120° C. for 24 hours, thus completing a first electrode.

A porous expanded polytetrafluoroethylene membrane-support film(thickness—20 micrometers; pore volume—93%; IBP—0.15 kg/cm²) was fixedto the ion exchange resin/catalyst impregnated surface of the firstelectrode. The porous PTFE film was coated by brush with a liquidcomposition of isopropyl alcohol containing 5 wt. % Nafion®perfluorosulfonic acid resin (manufactured by DuPont Co.), and airdried. The coating and air drying steps were repeated 3 times until thepores of the PTFE film were essentially completely filled and the layersjoined by the solid polymer ion exchange resin present in each layer,thus forming a first unitary assembly of the invention.

An aqueous dispersion of carbon black particles (“Denka Black”, suppliedby Denka Co.) and PTFE resin particles having a solids concentration 60wt. % carbon black and 40 wt. % PTFE was prepared. The PTFE wascoagulated, and the coagulum of mixed carbon black and PTFE dried.Naphtha was added and mixed into the dried coagulum as a lubricant. Thelubricated coagulum was ram-extruded to form a tape 2.5 mm thick. Theextruded tape was calendered and the thickness reduced to about 300micrometers. The calendered tape was uniaxially stretched (in thelongitudinal direction) at a temperature of about 250° C. to 5 times itsoriginal length, and then again calendered to reduce its thickness by afactor of 5. The electrically-conductive gas permeable electrode sheetthus produced was about 60 micrometers thick, had a nominal pore size ofabout 1 micrometer, and a bulk density of 0.51 g/cc. A collector sheet,identical to the collector sheet bonded to the first electrode, wasfixed to one surface of the gas permeable electrode sheet.

A porous expanded polytetrafluoroethylene electrode-support film,identical to the electrode-support film of the first electrode, wasfixed to the other surface of the gas permeable electrode sheet, andimpregnated with the liquid mixture of ion exchange resin/catalystparticles, and heat treated as described above, thus forming a secondelectrode.

A porous expanded polytetrafluoroethylene membrane-support film,identical to the membrane-support joined to the first electrode, wasfixed to the ion exchange resin/catalyst impregnated surface of thesecond electrode, and was impregnated with the same materials and in thesame manner, thus forming a second unitary assembly of the invention.

A small amount of the same liquid composition of isopropyl alcoholcontaining 5 wt. % Nafion® perfluorosulfonic acid resin described abovewas then applied by brush to the surface of the Nafion® resin-filledmembrane-support film of the second assembly and the ion exchangeresin-containing surfaces of the first and second assemblies werebrought together and lightly pressed to remove entrapped air andintimately join the assemblies, after which the solvent was removed byair drying and another embodiment of the unitary assembly of theinvention completed.

This embodiment of the invention was mounted and operated as a gaseousfuel cell. Humidified hydrogen was fed on one side of the mountedassembly, and oxygen was fed on the other side at an operatingtemperature of 80° C. The cell developed a voltage of 0.71 volts at acurrent level of 1 A/cm².

EXAMPLE 9

A porous expanded polytetrafluoroethylene membrane-support film(thickness—10 micrometers; pore volume—83%; IBP—1.75 kg/cm²) wassuperposed on a polypropylene release sheet. The membrane-support filmwas coated by brush with a liquid composition of isopropyl alcoholcontaining 5 wt. % Nafion® perfluorosulfonic acid resin (manufactured byDuPont Co.), and air dried at 70° C. The coating and air drying stepswere repeated 4 times until the pores of the PTFE film were essentiallycompletely filled, resulting in a virtually transparent composite solidpolymer ion exchange membrane.

A liquid mixture containing 25 wt. % Nafion® perfluorosulfonic acidresin (manufactured by DuPont Co.) and 75 wt. % carbon black/platinum(30 wt. %) particles (from NE Chemcat Co.) was prepared as described inExample 8.

A porous expanded polytetrafluoroethylene electrode-support film(thickness—10 micrometers; pore volume—91%; IBP—0.13 kg/cm²) was fixedto the surface of the composite solid polymer ion exchange membrane onthe polypropylene release sheet. The liquid mixture was applied by brushto the surface of the electrode-support film, impregnated into the poresof the film, and joined to the PTFE membrane-supported solid polymer ionexchange membrane. The solvent was removed by air drying at 70° C., anda unitary assembly of the invention was produced.

A second assembly was formed in like manner. The two assemblies werebrought together with the ion exchange membrane surfaces in contact,thermally fused together by passage through the nip of a pair of rollsheated to 150° C., and removed from the polypropylene release sheet,thus forming another embodiment of the invention having a unitaryassembly in an electrode/solid polymer ion exchange membrane/electrodearrangement.

A porous gas permeable electrode sheet and a PTFE-treated carbon papercollector, of the materials and arranged as described in Example 2, weredisposed against each electrode surface of the electrode/solid polymerion exchange membrane/electrode embodiment and held in place in a fuelcell by mechanical force exerted against them by ribbed gas supplyseparators. In the fuel cell, one electrode served as the air electrode,while the other served as the hydrogen electrode. When the fuel cell wassupplied with air and hydrogen, an output of 0.64 volts at a currentdensity of 0.5 A/cm² was obtained. The alternating current resistancewas about 0.07 ohm-cm², and there was virtually no change in theresistance after 800 hours of operation.

EXAMPLE 10

Two unitary solid polymer ion exchange membrane/electrode assemblieswere made as described in Example 9, except that the porous expandedpolytetrafluoroethylene membrane-support film fixed on the polypropylenerelease sheet was 6 micrometers thick and only two impregnation/dryingsteps used to impregnate and fill the film with the liquid compositionof isopropyl alcohol containing 5 wt. % Nafion® perfluorosulfonic acidresin.

A separately prepared Nafion® 112 perfluorosulfonic acid ion exchangemembrane (manufactured by DuPont Co.) was interposed between the ionexchange membrane surfaces of the unitary assemblies and bonded by hotpressing at a temperature of 140° C. and pressure of 30 kg/cm² to form aunitary electrode/solid polymer ion exchange membrane/electrode assemblyof the invention.

This embodiment of the invention was placed in a fuel cell having thearrangement described in Example 9 and, when supplied with oxygen andhydrogen, the fuel cell had an output of 0.6 volts at a current densityof 1 A/cm².

EXAMPLE 11

A liquid mixture containing 25 wt. % Nafion® perfluorosulfonic acidresin (manufactured by DuPont Co.) and 75 wt. % carbon black/platinum(30 wt. %) particles (from NE Chemcat Co.) was prepared as described inExample 8.

A porous expanded polytetrafluoroethylene electrode-support film(thickness—10 micrometers; pore volume—91%; IBP—0.13 kg/cm²) was placedon the surface of a polypropylene release sheet. The liquid mixture wasapplied by brush to the surface of the electrode-support film andimpregnated into the pores of the film. The solvent was removed by airdrying at 70° C., and a first electrode was completed.

A porous expanded polytetrafluoroethylene membrane-support film(thickness—18 micrometers; pore volume—85%; IBP—1.75 kg/cm²) wassuperposed on the electrode. The membrane-support film was coated bybrush with a liquid composition of isopropyl alcohol containing 9 wt. %Nafion® perfluorosulfonic acid resin (manufactured by DuPont Co.), andair dried at 70° C. The coating and air drying steps were repeated 5times until the pores of the membrane-support film were essentiallycompletely filled, and the ion exchange membrane thus formed was joinedto the electrode.

A porous expanded polytetrafluoroethylene electrode-support film as usedin the first electrode was placed on the ion exchange membrane of theassembly described above. A second electrode was formed and joined tothe ion exchange membrane in the same manner and with the same materialsas the first electrode. After the final drying step, the unitaryelectrode/solid polymer ion exchange membrane/electrode assembly of theinvention was removed from the polypropylene release sheet.

This embodiment of the invention was used to construct a fuel cellhaving the arrangement described in Example 9 and, when supplied withair and hydrogen, the fuel cell had an output of 0.62 volts at a currentdensity of 0.5 A/cm². The alternating current resistance of the fuelcell was 0.065 ohm-cm².

1. A unitary assembly for an electrochemical cell comprising: anelectrode having at least one electrically conductive material and amembrane support comprising polytetrafluoroethylene, said support beingimpregnated with a polymer ion exchange resin and having a surface, anda layer of ionomer joined to the electrode at said surface having apolymer ion exchange resin.
 2. A unitary assembly according to claim 1,in which the polytetrafluoroethylene of the electrode is expandedpolytetrafluoroethylene.
 3. A unitary assembly according to claim 1, inwhich said layer of ionomer comprises a polytetrafluoroethylene support.4. A unitary assembly according to claim 3, in which saidpolytetrafluoroethylene support comprises: a composite nonporous solidpolymer ion exchange membrane having first and second planar surfaces,said ion exchange membrane comprising at least one preformedmembrane-support film of porous expanded polytetrafluoroethylene havinga plurality of micropores, and a solid polymer ion exchange resinessentially completely filling the micropores of the membrane supportfilm.
 5. A unitary assembly according to claim 3 in which saidpolytetrafluoroethylene support of said layer of ionomer is preformed.6. A unitary assembly according to claim 3, in which saidpolytetrafluoroethylene support of said layer of ionomer has a thicknessof 1 to 100 micrometers.
 7. A unitary assembly according to claim 3, inwhich said polytetrafluoroethylene support of said layer of ionomer hasa pore volume of 60 to 98 percent.
 8. A unitary assembly according toclaim 3, in which said polytetrafluoroethylene support of said layer ofionomer has a thickness of 1 to 100 micrometers and a pore volume of 60to 98 percent.
 9. A unitary assembly according to claim 3, in which saidporous polytetrafluoroethylene support of said layer of ionomer is madenonporous by said solid polymer ion exchange resin.
 10. A unitaryassembly according to claim 1, in which said layer of ionomer comprisesa porous expanded polytetrafluoroethylene support.
 11. A unitaryassembly according to claim 1 in which the polytetrafluoroethylene ofthe electrode is preformed.
 12. A unitary assembly according to claim 1,in which the polytetrafluoroethylene of the electrode is aparticle-filled expanded polytetrafluoroethylene film.
 13. A unitaryassembly according to claim 1, in which said electrically conductivematerial comprises a catalyst.
 14. A unitary assembly according to claim13, wherein said catalyst is selected from the group consisting of leaddioxide, platinum and platinum alloys, and mixtures thereof.
 15. Aunitary assembly according to claim 14 in which said platinum andplatinum alloys catalysts are supported on carbon black.
 16. A unitaryassembly according to claim 13, in which said membrane support comprisesa polytetrafluoroethylene membrane impregnated with a liquid dispersionof noncatalytic electrically-conductive particles and a liquidcomposition containing a precursor of said catalyst and said polymer ionexchange resin.
 17. A unitary assembly according to claim 13, in whichsaid catalyst precursor is bonded to exchange groups of said polymer ionexchange resin.
 18. A unitary assembly according to claim 13, in whichsaid noncatalytic electrically-conductive particles comprise carbonblack.
 19. A unitary assembly according to claim 13, in which saidcatalyst precursor comprises a platinum-ammine complex solution.
 20. Aunitary assembly according to claim 13, in which said catalyst iscontained in the polytetrafluoroethylene of the electrode.
 21. A unitaryassembly according to claim 1, in which the electrode and said layer ofionomer are adhesively joined by said polymer ion exchange resin.